Present-day injection-molding technology enables the simultaneous production of a group of thermoplastic preforms in as many mold cavities in an operating cycle of about 20 seconds. Upon discharge from the mold cavities, however, these preforms are still hot and therefore not sufficiently self-supporting to permit their transportation to an associated blow-molding station without prior cooling. Thus, the body of the preform--which is expanded in the blow-molding process--is generally of considerable wall thickness, e.g. on the order of 4 to 6 mm, thereby storing a substantial quantity of heat; this is not the case with the neck of the preform which does not undergo expansion and whose wall thickness therefore corresponds to that desired in the final product. If simple air cooling is used to solidify the preforms or parisons until they can be transported, their accumulation in the vicinity of the injection-molding machine would extend the operating cycle of that machine by about 50% with corresponding reduction of the production rate.
Suitable thermoplastic materials for the two-stage production of bottles for carbonated beverages or the like include polyesters, nitriles and polypropylene.